Night Shining Clouds imaged by Jon Davies a few miles SW of Glasgow, Scotland on 18th July '06. ©Jon Davies, shown with permission.

Noctilucent clouds are Earth's highest at 50-53 miles just below the mesopause, the coldest part of the atmosphere. They are visible in summer at moderate to high latitudes when the mesopause is at its coldest. Their very small (1/10,000 mm) ice crystals scatter the sun's rays still shining at their altitude long after sunset on the ground. They appear blue because some of their light is absorbed by ozone lower in the stratosphere. NLCs "might" be becoming more frequent. But a factor might be that more and more sky watchers are looking out for them!    More on NLCs        NLC gallery